Amitava Majumder

Following is the basic structure of a C program.

Documentation Consists of comments, some description of the program, programmer name and any other useful points that can be referenced later.
Link Provides instruction to the compiler to link function from the library function.
Definition Consists of symbolic constants.
Global declaration Consists of function declaration and global variables.
main( )
{

} Every C program must have a main() function which is the starting point of the program execution.
Subprograms User defined functions.
Lets explore the sections with an example.

Write a program to print area of a circle.
In the following example we will find the area of a circle for a given radius 10cm.

Formula
The formula to compute the area of a circle is πr2 where π is PI = 3.1416 (approx.) and r is the radius of the circle.

Lets write the C code to compute the area of the circle.

#include <stdio.h>

#define PI 3.1416

float area(float r);

int main(void)
{
float r = 10;
printf("Area: %.2f", area(r));
return 0;
}

float area(float r) {
return PI * r * r;
}
The above code will give the following output.

Area: 314.16
Different sections of the above code
Documentation
This section contains a multi line comment describing the code.

In C, we can create single line comment using two forward slash // and we can create multi line comment using /* */.

Comments are ignored by the compiler and is used to write notes and document code.

Link
This section includes header file.

#include <stdio.h>
We are including the stdio.h input/output header file from the C library.

Definition
This section contains constant.

#define PI 3.1416
In the above code we have created a constant PI and assigned 3.1416 to it.

The #define is a preprocessor compiler directive which is used to create constants. We generally use uppercase letters to create constants.

The #define is not a statement and must not end with a ; semicolon.

Global declaration
This section contains function declaration.

float area(float r);
We have declared an area function which takes a floating number (i.e., number with decimal parts) as argument and returns floating number.

main( ) function
This section contains the main() function.

int main(void)
{
float r = 10;
printf("Area: %.2f", area(r));
return 0;
}
This is the main() function of the code. Inside this function we have created a floating variable r and assigned 10 to it.

Then we have called the printf() function. The first argument contains "Area: %.2f" which means we will print floating number having only 2 decimal place. In the second argument we are calling the area() function and passing the value of r to it.

Subprograms
This section contains a subprogram, an area() function that is called from the main() function.

float area(float r) {
return PI * r * r;
}
This is the definition of the area() function. It receives the value of radius in variable r and then returns the area of the circle using the following formula PI * r * r.

The math.h header defines various mathematical functions and one macro. All the functions available in this library take double as an argument and return double as the result. 

string.h is the header in the C standard library for the C programming language which contains macro definitions, constants and declarations of functions and types used not only for string handling but also various memory handling functions;